1. Field of the Invention
The present invention relates to an electronic flash device for use on a camera for flash photography, and more particularly it relates to an improved type of electronic flash device capable of pre-emission of flash light before a flash exposure is effected.
2. Description of the Prior Art
There have been proposed exposure control systems wherein in flash photography, an electronic flash tube is fired immediately before a film exposure and the light reflected from an object being illuminated is measured. The measured value is stored to control the amount of light to be emitted from the same or another electronic flash tube at the time of actual exposure. Flash devices used in such exposure control systems have relatively short flash times of one to two milli-seconds for both pre-emission of light and flash exposure. Such flash time for pre-emission light is too short if the pre-emission light is to be used for light measurement values for automatic diaphragm aperture control, or used for detecting a camera-to-object distance for automatic focus control of a camera objective.
U.S. Pat. No. 3,846,811 discloses circuitry for obtaining constant flash light output during normal flash photography. There is no suggestion in the reference to utilize such circuitry for pre-emission flash light.